Why Does PCB Bubbling Occur? How to Solve It?

In the field of electronic manufacturing, the PCB, known as the “mother of electronic components,” directly determines the reliability and service life of end products. PCB bubbling is regarded as an “invisible killer” during production and usage—mild cases lead to poor circuit contact and obstructed signal transmission, while severe cases may cause short circuits and burn-outs, resulting in significant rework costs and brand losses for enterprises. Whether you are an engineer in an SMT workshop or a procurement manager of electronic equipment, this tricky problem is unavoidable. Today, we will break down the core logic of PCB bubbling from three perspectives—“What it is, why it happens, and what to do about it”—and provide a practical set of solutions.

Forms of PCB Bubbling

PCB bubbling is not a single phenomenon. Depending on the bubbling location, shape, and formation stage, it can be categorized into various types. Accurate identification is the foundation for effective problem-solving.

1. Classified by Bubbling Location

• Bubbling between substrate and copper foil:
  The most common type. It appears as a hollow bulge between the copper foil and the substrate (such as FR-4). Pressing with fingers gives slight elasticity. In severe cases, the copper foil detaches with the bulge, directly damaging circuit continuity.

• Solder mask bubbling:
  The solder mask ink separates from the substrate or copper surface, forming transparent or yellowish bulges. This affects insulation performance and causes surface defects that cannot pass customer visual inspection.

• Pad bubbling:
  Localized bulges on pad areas, usually occurring after soldering, leading to weak or false solder joints—one of the main causes of later product failures.

• Internal bubbling in multilayer boards:
  Hidden inside multilayer PCBs and difficult to detect early. They typically appear during high-temperature operation or reliability testing. Repair is extremely difficult and generally results in PCB scrap.

2. Classified by Formation Stage

• Bubbling during production:
  Occurs directly during processes such as lamination, curing, or soldering, mostly related to process parameters or material quality. Damage can be stopped in time.

• Bubbling during storage/transportation:
  Occurs after PCB fabrication due to poor storage environments or transport vibration—often overlooked but very costly.

• Bubbling during end-use:
  Appears during product operation under heat, humidity, or vibration, causing direct product failure and serious brand damage.

Core Causes of PCB Bubbling

Although PCB bubbling appears to be a “surface problem,” it is in fact the concentrated outbreak of issues related to raw materials, manufacturing processes, and environmental control. Only by identifying the root cause can the issue be solved precisely.

1. Raw Materials: “Innate Defects” as the Root Hazard

• Substrate quality issues:
  Resin content, moisture content, and heat resistance of substrates like FR-4 are critical.

  • Low resin content → insufficient adhesion

  • Excessive moisture (usually ≤0.05% required) → vaporizes under heat and pushes up copper foil or solder mask

  • Poor heat resistance → softens or decomposes during soldering, losing adhesion

• Copper foil problems:
  Poor roughness, adhesion, or surface contamination/oxidation reduce bond strength. Under heat, separation occurs easily. Electrolytic copper foil with inadequate surface treatment is even more prone to bubbling.

• Solder mask ink defects:
  Poor adhesion or heat resistance, or moisture absorption during storage, prevents proper bonding after printing. Incorrect mixing ratios (e.g., hardener amounts) lead to incomplete curing and bubbling risk.

2. Manufacturing Process: “Operation Deviations” as the Direct Driver

(1) Pre-treatment: Insufficient cleaning → no adhesion foundation
Oil stains, oxidation, or dust on PCB surfaces block bonding.

• Oxidized copper forms a loose oxide layer preventing resin adhesion

• Dust on substrate creates “barriers,” causing localized solder mask bulging

(2) Lamination: Parameter miscontrol → weak interlayer bonding
Precise control of temperature, pressure, and time is essential.

• Too low temperature: resin can’t flow or fill gaps

• Too high temperature: resin decomposes

• Insufficient pressure: air trapped inside forms bubbles

• Excess pressure: resin squeezed out, reducing bonding area

• Improper curing time: too short → incomplete cure; too long → resin aging

(3) Solder mask printing/curing: Poor processing → hidden risks

• Uneven squeegee pressure or excessive speed → uneven thickness, bubbles

• Insufficient pre-bake → solvent not fully evaporated → bubbles during curing

• Improper curing temperature/time → incomplete cross-linking, weak adhesion

• Rapid temperature changes → thermal stress → later bubbling

(4) Soldering: High-temp shock triggers weak points
SMT reflow or wave soldering at 200–260°C stresses PCB materials.
Weak bonding areas expand and separate from thermal expansion mismatch, forming bubbles. Over-temperature or long dwell time worsens resin decomposition.

3. Environment & Storage: Poor “Post-Care” Causes Delayed Issues

PCBs require strict temperature/humidity control (ideal: 20–25°C, 40–60% RH).
Moisture absorption, thermal cycling, or poor packaging during transportation lead to bubbling.

4. Design Defects: Hidden “Innate Loopholes”

Design flaws may cause bubbling, including:

• Large copper areas without thermal relief → overheating during soldering

• Overlapping inner-layer copper → trapped air during lamination

• Poor solder mask–copper edge transitions → easier delamination

Solutions for PCB Bubbling in Different Scenarios

1. Bubbling During Production: Stop Loss Quickly, Optimize Processes

• Substrate–copper foil bubbling:

  • Check moisture content (via baking test)

  • Replace defective materials

  • Recalibrate lamination parameters

  • Light bubbling → secondary lamination; severe → scrap

• Solder mask bubbling:

  • Before curing: remove ink → re-treat surface → re-print → cure properly

  • After curing: small areas → repair; large areas → rework + root cause analysis

• Soldering bubbling:

  • Pause soldering

  • Check temperature profile

  • Reduce temperature/dwell time

  • Pre-bake moisture-absorbed PCBs

2. Bubbling During Storage/Transport: Improve Environment, Strengthen Protection

• Bake bubbled PCBs (50–60°C for 2–4 hours) and inspect.

• Enhance storage humidity control systems.

• Use vacuum packaging + desiccants.

• Improve anti-vibration and moisture protection during transport.

3. Bubbling During End-Use: Trace the Root Cause, Rectify Fully

• Recall affected products

• Analyze bubbling location and cause

• If raw materials → change suppliers

• If process → inspect parameters, retrain operators

• If design → redistribute layouts or add thermal structures

• Build customer feedback loop to track improvement results

4. General Repair Techniques: Emergency Fix for Small-Area Bubbles

Suitable only for non-critical areas:

1. Carefully cut open the bubble surface

2. Clean with anhydrous alcohol

3. Apply PCB repair adhesive

4. Cure in an oven per adhesive specs
   Large or critical-area bubbling still requires scrapping.